1. Field of the Invention
The present invention relates to paper-making, and more particularly refers to the production of a composite paper particularly well adapted for use as cover sheets in the production of gypsum wallboard.
2. Description of the Prior Art
Paper for gypsum board is conventionally made by pulping up waste paper constituents of old corrugated paper, or kraft cuttings and waste news. In cleaning, screening and refining the suspended materials in water suspension, the process paper stock is diluted still further with water and then formed by draining the plies of paper on several continuously moving wire cylinders, where the separate plies are joined together by a carrying felt. The weak paper web is then dewatered in a press section where water is pressed out of the web. The pressed paper is dried in a multi-cylinder drying section with steam added to each cylinder. The dried paper is subjected to a squeezing or calendaring operation for uniformity in thickness and is then finally wound into rolls. The paper is subsequently utilized as paper cover sheets to form gypsum wallboard by depositing a calcined gypsum slurry between two sheets, and permitting the gypsum to set and dry.
Conventional paper used in gypsum wallboard has definite limitations with regard to the utilization of heat energy. First, it has definite drainage limitations in forming and pressing, and additional limitations in the drying rate. The drainage rate limitations impose a large paper drying energy load on the mill. Additionally, because the paper is not sufficiently porous, it takes a greater heat energy load to dry the finished gypsum wallboard subsequent to its formation. It would be highly desirable to have a more porous paper for utilization as paper cover sheets in the formation of gypsum wallboard to permit the achievement of a substantial reduction in drying energy load, while still having a paper which has the requisite physical properties with regard to physical strength.
In U.S. Pat. No. 4,225,383, there is disclosed a paper formulation whose purpose is designed to avoid the use of asbestos fibers. The composition comprises from 1% to about 30% fibers, from about 60% to about 95% inorganic filler and from about 2% to about 30% of a film-forming latex. The paper is stated as being designed as a replacement or substitute for asbestos fibers in such applications as for making muffler paper, underlayment felt for vinyl floor covering, gasket papers, roofing paper, sound-deadening paper, pipe wrap, insulation paper, heat deflection papers, cooling tower packing, electrically resistant paper and board products. Papers having the disclosed composition were fabricated, and attempted to be used as cover sheets for making gypsum wallboard by the present inventors. However, although the material proved to have good porosity, the tensile strength of the paper was far to low to be utilized for making gypsum wallboard.